1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery employing a positive electrode, a negative electrode, and a non-aqueous electrolyte comprising an electrolyte dissolved in a non-aqueous solvent. More particularly, the invention relates to a non-aqueous electrolyte secondary battery employing a negative electrode using graphite as a negative electrode active material and a non-aqueous electrolyte using a non-aqueous solvent containing propylene carbonate, which is characterized in that reaction between propylene carbonate and the graphite of the negative electrode active material is restricted so that excellent charge/discharge cycle performances can be attained.
2. Description of the Related Art
Conventionally, as one of advanced secondary batteries featuring high power and high energy density, non-aqueous electrolyte secondary batteries employing non-aqueous electrolytes wherein lithium ion is moved between their positive electrodes and negative electrodes to perform charging and discharging have been commonly used.
Further, recently, such non-aqueous electrolyte secondary batteries have been under consideration for utilizing in various fields, such as a power source of electric automobiles.
As the non-aqueous electrolyte secondary batteries have been utilized in various fields, such as the power source of electric automobiles, demands for higher power performance under low temperature conditions and higher preservation durability under high temperature conditions have been increasing.
In the non-aqueous electrolyte secondary batteries, propylene carbonate having a lower melting point than that of ethylene carbonate has been used as a non-aqueous solvent of the non-aqueous electrolytes for improving power performance under low temperature conditions.
However, these non-aqueous electrolyte secondary batteries employing propylene carbonate as the non-aqueous solvent of the non-aqueous electrolytes have a problem as follows. When graphite having a high charge/discharge efficiency and a high energy density is used as a negative electrode active material in the negative electrodes, propylene carbonate reacts with the graphite and decomposes, resulting in drawbacks that the charge/discharge efficiency and so on of the non-aqueous electrolyte secondary batteries are lowered and sufficient battery performances are so difficult to be obtained.
Therefore, in recent years, in order to prevent propylene carbonate from reacting and decomposing, there has been many proposals to use low crystalline carbon coated graphite wherein the surface of graphite is coated with a low crystalline carbon material having lower crystallinity than that of graphite, as the negative electrode active material (see, Japanese Publication Nos. 5-121066, 9-237638, and 9-326253).
Nevertheless, a problem in using the low crystalline carbon coated graphite wherein the surface of graphite is coated with the low crystalline carbon material having lower crystallinity than that of graphite as the negative electrode active material, has been that, because a volume change of the graphite during charge/discharge cycling is different from that of the low crystalline carbon coating its surface, the low crystalline carbon coating the surface is peeled off the graphite and propylene carbonate reacts with thus exposed graphite and decomposes in the case of long repeated charge/discharge cycling or charging/discharging under high temperature conditions, so that sufficient charge/discharge cycle performances are so difficult to be obtained.